It is generally preferable to lower the core loss of a grain-oriented electrical steel sheet, from a point of energy conservation. Japanese Patent Publication No. S58-26405 describes a method for lowering core loss, in which magnetic domains are fractionalized by laser irradiation. This method aims to achieve a lower core loss by introducing a stress strain to a grain-oriented electrical steel sheet by the reactive force of thermal shock waves generated by the irradiation of a laser beam, and thus by fractionalizing magnetic domains. However, a potential problem of such method is that the strain introduced by the laser irradiation disappears during annealing, and therefore the effect of fractionalizing the magnetic domains is significantly reduced, or even lost. Therefore, even though this method may be applied to a grain-oriented electrical steel sheet for a laminated core transformer which does not require stress-relieving annealing, it is generally not applicable to a steel sheet for a wound core transformer that requires stress-relieving annealing.
In order to improve the core loss of a grain-oriented electrical steel sheet by enabling the effect of lowering core loss to be retained even after stress-relieving annealing, various conventional methods have been provided in which a deformation exceeding a stress strain level may be imposed on a steel sheet, whereby the magnetic permeability thereof can be changed and, by so doing, the magnetic domains can be fractionalized. Such methods may include a method with which groove-shaped or pit-shaped concaves may be formed on the surface of a steel sheet by pressing the steel sheet with a tooth roll (as described in Japanese Patent Publication No. S63-44804); a method in which concaves may be formed on the surface of a steel sheet by chemical etching (as described in U.S. Pat. No. 4,750,949); and a method in which grooves comprising the lines of pits may be formed on the surface of a steel sheet with a Q-switched CO2 laser (as described in Japanese Patent Publication No. H7-220913). Another example of such method is one in which instead of grooves, melted and re-solidified layers may be formed on the surface of a steel sheet with a laser (refer to Japanese Patent Publication Nos. 2000-109961 and H6-212275).
Some of the problems of the conventional methods mentioned above are that, e.g., (i) in the case of the mechanical-type method in which a tooth roll is used, maintenance is need to be performed frequently because the electrical steel sheet is hard, and thus the teeth of the roll may likely be worn away in a short span of time; (ii) in the case of the method in which chemical etching is employed, while it may not have the problem of the wear of the teeth, the steps of masking, etching and removing the mask are needed, and thus these processes are more complicated than a mechanical method; (iii) in the case of the method in which grooves that include lines of pits are formed on the surface of a steel sheet with a Q-switched CO2 laser, though such method does not have the problem of the wear of the teeth or the complication of previously described processes (because the concaves are formed in a non-contact manner), a special Q-switching device should be added separately to a commercially available laser oscillator; (iv) in the case of the method with which grooves are formed, because some parts of the steel sheet are removed, the space factor thereof is likely lowered, thus adversely influencing the performance of a transformer; and, (iv) in the case of the method with which melted and re-solidified layers are formed, though the lowering of a space factor is either reduced or eliminated, core loss is not sufficiently improved.
The entire disclosures of the referenced publications are incorporated herein by reference.